This invention relates to pressure indicating devices and particularly but not exclusively to such devices for indicating vehicle tyre pressure.
A wide variety of pressure indicating devices have been known for some time, the majority being devices which provide a means to accurately measure pressure, particularly of gases and liquids.
In certain situations, a precise measurement of pressure is not required but rather a simple and clear indication of a change in pressure is desired, for example to detect leakage from a pressurised container. In such situations, a device which provides a highly visual xe2x80x9csee-at-a-glancexe2x80x9d indication that leakage is occurring is more appropriate than a precise numerical measurement of the pressure within the container.
Such devices are well known in the art. For example, a device is known for indicating whether a pneumatic circuit is pressurised, available under the trade mark ROTOWINK (ex Norgren Martonair Limited). This device is adapted to be mounted on a control panel, with the inlet connected to part of a pneumatic circuit, e.g. a pressure vessel, such as a compressor reservoir. The device contains a flexible, impermeable diaphragm that spans the inlet. The diaphragm supports a spring returnable piston.
A rod and crank arrangement extends from the piston, terminating in the head of the device, remote from the inlet, where the rod is attached to a crank within a rotatable ball having its opposite halves different colours, e.g. red and green. The ball is mounted beneath a lens through which one side of the ball is clearly visible from the exterior of the device.
A difference in pressure across the diaphragm, e.g. resulting from a build up of air pressure in a compression chamber to which the inlet of the device is connected, causes the diaphragm to flex towards the head of the device, which pushes the piston and rod towards the head. This movement causes the crank to cause the ball to rotate such that the other side of the ball is then visible through the lens. This other ball side of contrasting colour provides a clear visual signal that pressure has built up in the chamber.
It is highly desirable for vehicle tyres to remain at their optimum pressure for reasons of safety, fuel economy and comfort and to prolong the life of the tyres. Consequently, it is preferable that tyre pressures are checked at regular intervals. However, this mundane task is frequently overlooked since a lower than optimal tyre pressure, such as caused by a slow puncture or an inaccurate gauge on a pump or compressed air supply unit, is only perceived when the tyre looks dangerously flat and/or the vehicle feels uncomfortable, at which stage the tyre pressure is well below its optimum. Clearly, in the period between initial loss of air pressure and detection, any use of the vehicle reduces the life of the tyre significantly and will have put the driver and passengers of the vehicle at some risk.
Although it occurs less often, it is also potentially dangerous to drive a vehicle with one or more of its tyres grossly over inflated rather than under inflated.
Attempts have been made in the past to provide tyre pressure monitoring devices. U.S. Pat. No. 4,814,745 describes a cap for attachment to a tyre valve following inflation and which includes an alarm circuit which is de-energised when the tyre is inflated to proper pressure. The cap, however, must include its own power supply and other circuit members which is undesirably complex, prone to failure and expensive.
U.S. Pat. No. 4,819,686 discloses a purely mechanical device including a diaphragm in a chamber. The diaphragm may be deflected to give a visual indication of the state of pressure within the tyre. One side of the diaphragm is a so-called reference chamber which, when the device is applied to a tyre valve stem, is pressurised to the pressure within the tyre. Thereafter the reference chamber is sealed while the other side of the diaphragm is exposed to the pressure within the tyre, so that if the latter drops, the diaphragm moves. The construction is relatively complex and the cap provides no indication as to whether the tyre is inflated to the correct pressure. The device is only effective to show a drop in pressure over time. British patent specification 2277802 discloses a similar device but in which an indication of the state of pressure in the tyre is achieved only with the use of a hand held device adapted to be brought into close proximity with the cap on the valve stem. It suffers from the same disadvantage as noted above.
U.S. Pat. No. 2,689,481 describes a tyre pressure indicating device comprising a housing which may be screwed on to a tyre valve stem and which includes an inlet for air under pressure from the tyre into a transparent walled chamber. The chamber contains a hermetically sealed bellows, the degree of compression of which changes its length to provide, in conjunction with markings on the exterior of the chamber, an indication of tyre pressure.
According to the present invention there is provided a pressure indicating device comprising a housing which includes an inlet for air under pressure, a permanently hermetically sealed chamber including a gas under pressure, a flexible member between the inlet and the chamber, and a signalling device, the signal from which depends on the pressure difference between the inlet and the chamber, and characterised in that the flexible member is a flexible impermeable diaphragm which, as the pressure at the inlet increases, is caused to flex to extend into the pressurised chamber, and in that the device includes connecting means between the diaphragm and the signalling device adapted to actuate the signalling device to provide a signal representative of the inlet pressure relative to that in the sealed chamber. The signalling device is preferably located within the sealed chamber.
When such a device is to be used as a tyre pressure indicating device, the inlet may be adapted to form sealed contact with the valve when the device is fitted to the tyre valve stem in place of the conventional dustcap and, as it is screwed further on, to open the tyre valve so that the pressure in the inlet is the same as that in the tyre.
Preferably the chamber is pressurised to a pressure just below that of the optical tyre pressure for a particular vehicle, (the optimal tyre pressures being taken to be those recommended by the vehicle manufacturers). Consequently, when the device is connected to and opens the valve, the pressure differential between the higher pressure of the tyre and the lower pressure of the chamber causes the flexible impermeable diaphragm to flex, such that it extends into the pressurised chamber, and to actuate the signalling device to provide a signal representative of the tyre pressure relative to that in the sealed chamber.
The diaphragm construction may be a flat disc type or (and this may improve service life and sensitivity of the diaphragm) a rolling diaphragm.
Preferably the signalling device is capable of providing at least two visually distinguishable signals. This is most conveniently achieved by a mechanical means such as a rotatable mounted ball of which one side is one colour and the other a contrasting colour but as will be appreciated, any mechanical means of providing such signals may be substituted. The signal is preferably clearly visible over a wide range of viewing angles, e.g. a viewing angle of 110xc2x0 or more.
The means connecting the flexible impermeable diaphragm to the signalling device are preferably simple mechanical linkages capable of transmitting any movement of the diaphragm, in a manner which actuates the signalling device. Most preferably a spring returnable piston carrying an elongate member is mounted on the diaphragm, the end of the member being pivoted to a crank which is attached to a rotatable ball signalling device. In use, any flexing or flattening of the diaphragm will accordingly rotate the ball in one sense or the other.
In accordance with a particular preferred feature of the invention, the diaphragm may have a weakened section adapted to rupture if the pressure in the tyre exceeds that in the chamber by a predetermined amount. If such a diaphragm is used, when the device is fitted to an over inflated tyre, the indicator will briefly indicate adequate tyre pressure and then on rupture of the diaphragm, that all is not well.
The inlet of the device preferably has a screw threaded bore and at the end of the bore an axial nose which is positioned to depress the central pin of and therefore open, a conventional tyre valve. To ensure that the valve and device are in sealed contact and to prevent the escape of air from within the tyre, the inlet preferably contains an appropriately located O-ring.
A tyre pressure indicating device according to the present invention may thus easily be fitted onto any conventional tyre valve to display a clear visual signal which indicates the air pressure condition of the tyre. It may be mass produced inexpensively in quantity. It is also lightweight, self-contained and provides an indication of self-failure.